1. Technical Field
The technical field relates to a plasma display device that is known as a thin large-screen display device.
2. Background Art
Spontaneous light-emitting type display devices such as a plasma display device and a CRT display (Cathode-Ray Tube display) device are widely used since they do not have a viewing angle dependency and can display natural images. In particular, a plasma display device is thin and suitable for forming a large screen, and therefore is rapidly becoming widespread.
Conventionally, a plasma display device mainly includes a plasma display module section having a plasma display panel, a chassis conductor holding the plasma display panel, and various circuit boards attached to the chassis conductor, and a shield case surrounding the module.
This plasma display panel excites a phosphor provided in each discharge cell by an ultraviolet ray generated by gas discharge so as to emit visible light as display light. The plasma display panel includes a plurality of electrodes (scan-sustain electrode pairs and address electrodes) which are arranged in a lattice. By emitting light selectively in a discharge cell that is an intersection portion of the electrodes, an image is formed. With this principle, since a large current for driving flows in electrodes, an electromagnetic field is generated from a plasma display module due to this current.
Furthermore, a plasma display device is provided with various signal processing circuit boards for processing an image signal to be transmitted to a plasma display panel. An undesirable electromagnetic field due to a clock signal and the like used in these circuit boards may be generated.
A shield case for shielding from generated electromagnetic fields has a configuration in which, for example, a front glass to which a conductive filter is attached and a conductive back cover at the rear surface side are connected to each other by using a conductive member so as to surround a plasma display module. Thus, the plasma display module is electromagnetically shielded from generated electromagnetic fields.
However, with the increase in driving electric power due to recent improvements in image quality it has been difficult to reliably reduce an electromagnetic field by a conventional shield case. In particular, in low-frequency regions of not higher than several tens MHz, such an electromagnetic field cannot be sufficiently reduced by a conventional shield case and may be radiated to the outside as a noise.
In order to solve such a problem, the following proposition has been made. That is to say, an adjacent conductive cylinder is provided to a ground-return conductor plate connecting between a driving board provided at one end of a plasma display device and a driving board provided at the other end, in which inductance of the ground-return conductor plate is cancelled by an eddy current generated in this adjacent conductive cylinder. Such a proposition is disclosed in, for example, Japanese Patent Unexamined Publication No. 2001-83909.
Furthermore, a method for individually shielding by surrounding a board generating noise by a conductor has been proposed. Such a proposition is disclosed in, for example, Japanese Patent Unexamined Publication No. H10-17081.
Furthermore, a plasma display device having a configuration, in which shielding is carried out by connecting a chassis conductor holding a plasma display panel to a back cover so as to surround a driver circuit board, has been also proposed. Such a proposition is disclosed in, for example, Japanese Patent Unexamined Publication No. H10-282896.
However, in the plasma display device described in Japanese Patent Unexamined Publication No. 2001-83909, when an adjacent conductive cylinder having a size such that a reducing effect for cancelling inductance can be expected is inserted inside the panel and the ground-return conductor plate, an entire area of the loop of a flowing current is enlarged. As a result, electromagnetic fields themselves to be reduced are increased, thus deteriorating the effect of reducing electromagnetic fields.
Furthermore, in a conventional plasma display device, a driving current for driving a plasma display panel flows in a chassis conductor. Therefore, by a cable and the like extending to the outside of the shield case via various circuit boards connected to the chassis conductor, noise caused by the driving current is emitted from the outer side of the shield case. However, it is difficult to reduce this noise.
Furthermore, in plasma display devices disclosed in Japanese Patent Unexamined Publication Nos. H10-17081 and H10-282896, the shielding effect for signal processing circuit board and driver circuit board themselves is increased. However, it is difficult to reduce an electromagnetic field generated by a current flowing between the plasma display panel and the chassis conductor. Furthermore, it is difficult to reduce noise emitted from the outer side of the shield via a cable and the like.